First evidence of Alfven wave activity in KSTAR plasmas

We report on first evidence of wave activity during neutral beam heating in KSTAR plasmas: 40 kHz magnetic fluctuations with a toroidal mode number of n = 1. Our analysis suggests this a beta-induced Alfven eigenmode (BAE) resonant with the q = 1 surface. A kinetic analysis, when coupled with electron temperature measurements from electron cyclotron emission and ion/electron temperature ratios from crystallography, enables calculation of the frequency evolution, which is in agreement with observations. Complementary detailed magnetohydrodynamic (MHD) modelling of the magnetic configuration and wave modes supports the BAE conclusion, by locating an n = 1 mode separated from the continuum in the core region. Finally, we have computed the threshold to marginal stability for a range of ion temperature profiles. These suggest the BAE can be driven unstable by energetic ions when the ion temperature radial gradient is sufficiently large. Our findings suggest that mode existence could be used as a form of inference for temperature profile consistency in the radial interval of the mode, thereby extending the tools of MHD spectroscopy.X1132sciescopu

PREDICTIONS AND OBSERVATIONS OF GLOBAL BETA-INDUCED ALFVEN-ACOUSTIC MODES IN JET AND NSTX

In this paper we report on observations and interpretations of a new class of global MHD eigenmode solutions arising in gaps in the low frequency Alfven - acoustic continuum below the geodesic acoustic mode frequency. These modes have been just reported (Gorelenkov et al 2007 Phys. Lett. 370 70 - 7) where preliminary comparisons indicate qualitative agreement between theory and experiment. Here we show a more quantitative comparison emphasizing recent NSTX experiments on the observations of the global eigenmodes, referred to as beta-induced Alfven - acoustic eigenmodes (BAAEs), which exist near the extrema of the Alfven - acoustic continuum. In accordance to the linear dispersion relations, the frequency of these modes may shift as the safety factor, q, profile relaxes. We show that BAAEs can be responsible for observations in JET plasmas at relatively low beta 20%. In NSTX plasma observed magnetic activity has the same properties as predicted by theory for the mode structure and the frequency. Found numerically in NOVA simulations BAAEs are used to explain the observed properties of relatively low frequency experimental signals seen in NSTX and JET tokamaks.X1133sciescopu

Application of ICRF waves in tokamaks beyond heating

Interaction of waves in the ion cyclotron range of frequencies (ICRF) with a plasma has a number of key properties that make them attractive beyond pure heating. First, the waves can interact resonantly with either the plasma ions or electrons. In the case of ion cyclotron damping, a small number of resonant ions are often accelerated to high energies. These ions, apart from heating the bulk plasma via Coulomb collisions, can increase fusion reactivity, affect plasma stability and drive current. They have also been invaluable in diagnostic applications and simulations of fusion-born 3.5 MeV alpha-particles. The second key property of ICRF waves is the transfer of wave momentum to the plasma. This allows one to drive current, affect plasma rotation and induce radial transport of the fast-ions with toroidally directed waves. Finally, ICRF power deposition is rather narrow and its location can be externally controlled, which has important applications in improving the plasma performance, affecting the local plasma transport and providing a tool for plasma transport studies. Representative examples from present-day tokamak experiments are reviewed to highlight the available capabilities

Reversed shear Alfvén eigenmode stabilization by localized electron cyclotron heating

Reversed shear Alfvén eigenmode (RSAE) activity in DIII-D is stabilized by electron cyclotron heating (ECH) applied near the minimum of the magnetic safety factor (qmin) in neutral beam heated discharges with reversed-magnetic shear. The degree of RSAE stabilization, fast ion density and the volume averaged neutron production (Sn) are highly dependent on ECH deposition location relative to qmin. While discharges with ECH stabilization of RSAEs have higher Sn and more peaked fast ion profiles than discharges with significant RSAE activity, neutron production remains strongly reduced (up to 60% relative to TRANSP predictions assuming classical fast ion transport) even when RSAEs are stabilized. © 2008 IOP Publishing Ltd

Modelling third harmonic ion cyclotron acceleration of deuterium beams for JET fusion product studies experiments

Recent JET experiments have been dedicated to the studies of fusion reactions between deuterium (D) and Helium-3 (3He) ions using neutral beam injection (NBI) in synergy with third harmonic ion cyclotron radio-frequency heating (ICRH) of the beam. This scenario generates a fast ion deuterium tail enhancing DD and D3He fusion reactions. Modelling and measuring the fast deuterium tail accurately is essential for quantifying the fusion products. This paper presents the modelling of the D distribution function resulting from the NBI+ICRF heating scheme, reinforced by a comparison with dedicated JET fast ion diagnostics, showing an overall good agreement. Finally, a sawtooth activity for these experiments has been observed and interpreted using SPOT/RFOF simulations in the framework of Porcelli's theoretical model, where NBI+ICRH accelerated ions are found to have a strong stabilizing effect, leading to monster sawteeth